Paper currency and other types of bank notes typically include some form of deterrent against counterfeiting. Currency printers typically attempt to deter counterfeiting by giving the currency predefined magnetic signatures. The magnetic signature can be realized by using ink or dyes which have magnetic properties, for example, the ink can contain magnetized particles which produce a magnetic flux. The magnetic properties of the ink can be controlled so that there is a defined magnetic signature associated with authentic currency.
In the prior art, there are known devices which are used to validate paper currency and other notes by sensing the magnetic characteristic or signature. These devices utilize a magnetic head or sensor which contacts the bill and detects the magnetic field produced by the ink. Because the magnetic field can be weak, prior art validators typically include a pressure roller which squeezes the bill against the magnetic head. Through continual use the magnetic head can pick up dirt and other debris from the paper currency. Over time, this debris contaminates the magnetic head and degrades the performance of the validator unless the head is cleaned periodically. In addition, the requirement of bill contact to perform the validation process can reduce the ability of validator to handle worn out or damaged notes. Furthermore, because the prior art devices rely on the detection of a magnetic field in the bank note, an authentic, but demagnetized, bank note will not be validated by the prior art device.
Another problem encountered with prior art bank note validators is their suspectibility to non-intrusive tampering. There are known bank note validators which can be tricked into producing credit pulses when exposed to an electrostatic discharge, such as those produced by so-called "stun guns".